Development of Symptoms of Brown Rust of Barley in Relation to the Distribution of Fungal Mycelium, Starch Accumulation and Localized Changes in the Concentration of Chlorophyll

Summary The development of symptoms of brown rust (Puccinia hordei Otth.) of barley was examined in relation to the distribution of fungal mycelium within the leaf, the accumulation of chloroplastic starch and localized changes in the concentration of chlorophyll. Symptom development correlated exactly with the growth and distribution of mycelium within the leaf. Starch accumulated in cells associated with or directly surrounded by fungal mycelium, but rarely in cells beyond the leading edge of the colony. A photographic study of the development of a group of brown rust pustules from the flecking stage to the formation of green islands, using incident light and light transmitted through the leaf, suggested that the yellow colour of the flecks was not caused by chlorophyll breakdown, but that the major loss of chlorophyll occurred from regions between fungal pustules. This conclusion was supported by a study of the density of chlorophyll within different regions of the leaf (using the absorption of red light as a measure of chlorophyll density) and by extraction of chlorophyll from different regions of the leaf. These results are discussed in relation to the controversy surrounding the mode of formation of green islands.